Adult lung volumes depend on subject position, being less when supine than when standing. Two mechanisms may contribute to this change; 1) the lung may be compressed when the diaphragm is elevated; 2) the pulmonary blood volume may be increased, which, in turn, decreases compliance. In adult subject, this change in lung volume has also been associated with an increase in resistance and the change in compliance with an increase in closing volume. To determine whether this change in resistance would effect flow, we measured flow volume loops in young, non-obese subjects in whom closing volume should be minimal. We used flow-volume loops to obtain both a measure of lung volume and of flow (Forced Vital Capacity, FVC, and FEV1/FVC, the ratio of Forced Expired Flow in 1 second to FVC, respectively).
Flow-volume loops were performed in normal males ages 13 to 17 with BMI = 24.1 ± 3.8 (mean ± SD) who had no reported cardiopulmonary impairments. The Institutional Review Board approved this protocol. Measurements were made according to American Thoracic Society (ATS) standards in the standing and supine positions. By power analysis we determined that a sample size of 10 was sufficient to detect a 10% difference between the two positions when using the paired t-test, a P < 0.05, a power of 90% and a variability of 0.2 L. This variability was chosen because it is the ATS standard for reproducibility. The Bonferroni correction was used to adjust the P value for multiple variable comparisons, since two variables were measured, the FVC and the FEV1/FVC.
Values are mean ± S.D. in the tableStandingSupinePFVC4.51 ± 0.514.17 ± 0.490.0001FEV1/FVC84 ± 582 ± 80.15.
In young adult males, we confirmed the small (7%) change in FVC when changing position from standing to supine. However, there was no difference in the FEV1/FVC, indicating that this small change in volume is not enough to cause an increase in airway resistance detectable by spirometry.
Changes in lung volumes in young males do not effect flow.
C.A. McCuller, None.